Polymer composition with electrophilic groups for stabilization of lithium sulfur batteries

ABSTRACT

A polymer to be used as a binder for sulfur-based cathodes in lithium batteries that includes in its composition electrophilic groups capable of reaction with and entrapment of polysulfide species. Beneficial effects include reductions in capacity loss and ionic resistance gain.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional Application of U.S. patent applicationSer. No. 14/718,084, filed May 21, 2015 which, in turn, is aContinuation-in-Part Application of International Application NumberPCT/US14/62415, filed Oct. 27, 2014, which is a Non-ProvisionalApplication of U.S. Provisional Application No. 61/981,732, filed Apr.18, 2014 and U.S. Provisional Application No. 61/981,735, filed Apr. 18,2014, all of which are included by reference herein.

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates generally to lithium metal electrochemical cells,and, more specifically, to sulfur-based cathodes in lithium metalbatteries with polymeric electrolytes.

Sulfur-based materials are attractive cathode active materials forlithium batteries due to their high lithium capacities. For example, thetheoretical lithium capacity for elemental sulfur is 1675 mAh/g, andcapacities for sulfur compounds can be as high as 800 mAh/g or so, ascompared with capacities around 170 mAh/g for conventionally usedcathode materials such as lithium iron phosphate. However, lithiumbatteries that have sulfur-based cathodes tend to have poor cyclingstability due to the formation and migration of lithium polysulfidesalts (e.g., LiSx, 3<x<8) as well as the formation and/or diffusion ofelemental sulfur out of the cathode layer. These unboundsulfur-containing species separate from the cathode layer, causingirreversible capacity loss, and can migrate to the anode and decompose,causing an increase in internal ionic resistance of the cell or outrightdecomposition of the anode.

Lithium metal-based materials are attractive anode active materials forlithium batteries due to their high specific capacities of 3860 mAh/g.Coupling lithium metal anodes to sulfur-containing cathodes wouldprovide a very high specific capacity cell, and would result in a highspecific energy cell. However, stable cycling and safe operation ofbatteries containing lithium metal have proved elusive, no matter whatcathode material is used, due to either a reaction of the lithium metalwith the electrolyte or formation of lithium dendrites upon cycling.

Improvements in stability, cyclability and lifetime of lithium-sulfurbatteries are usually sought through the use of sulfur composites inwhich inactive materials are combined with sulfur to prevent diffusionof polysulfide and sulfur species. Examples include using carbonstructures or other molecular encaging species that can physicallyand/or chemically sequester sulfur and/or lithium polysulfides, or canreact with sulfur to form immobile species such as graphite or cyclizedPAN that chemically sequester the sulfur. Another example is usingsingle-ion conductors that allow transport of Li cations, but not anionsor elemental sulfur species. Examples of such single-ion conductorsinclude Li₃N, LISICON, LIPON, Thio-LISICON, Li₂S—P₂S₅, and the like.Suppression of lithium dendrites has been attempted by use of highmodulus electrolytes such as cross-linked PEO, block copolymerelectrolytes, and inorganic conductors.

What is really needed is a way to take full advantage of the highlithium capacity of sulfur-containing cathode materials coupled withlithium metal electrodes to make stable, long life cycle electrochemicalcells.

Lithium-sulfur couples have been studied as they have the potential toproduce batteries with higher capacity and higher energy thanconventional Li-ion batteries. However, there are many problems withthese systems. One problem is that sulfur is very soluble in typicalliquid electrolytes. In a conventional sulfur-based electrochemical cellsystem, the sulfur in the cathode (in the form of polysulfides, forexample) dissolves in the electrolyte and diffuses to the anode where itreacts with the lithium to form lithium sulfides. Trapped at the anodein the reduced state, the sulfur cannot be reoxidized to the originalform and be returned to the cathode. This leads to rapid capacity fadeand high impedance, resulting ultimately in cell death.

Another problem associated with lithium-sulfur systems arises from lossof surface area in the electrodes. During cycling, sulfur in theelectrode region aggregates into larger particles, permanently changingthe morphology of the cathode. The change in morphology results inreduced ionic and electronic conductivity. Thus it has not been possibleto produce viable battery systems from lithium-sulfur couples.

It would be useful to construct a battery in which sulfur could be usedas the active cathode material in order to exploit the high capacity andhigh energy that sulfur can provide.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and others will be readily appreciated by theskilled artisan from the following description of illustrativeembodiments when read in conjunction with the accompanying drawings.

FIG. 1A is a schematic drawing of a diblock copolymer molecule,according to an embodiment of the invention.

FIG. 1B is a schematic drawing that shows how multiple diblock copolymermolecules, as shown in FIG. 1A, arrange themselves into a domainstructure, according to an embodiment of the invention.

FIG. 1C is a schematic drawing that shows how multiple repeat domains,as shown in FIG. 1B, form a continuous nanostructured block copolymermaterial, according to an embodiment of the invention.

FIG. 2A is a schematic drawing of a triblock copolymer molecule thatincludes two different polymer blocks, according to an embodiment of theinvention.

FIG. 2B is a schematic drawing that shows how multiple triblockcopolymer molecules, as shown in FIG. 2A, arrange themselves into adomain structure, according to an embodiment of the invention.

FIG. 2C is a schematic drawing that shows how multiple repeat domains,as shown in FIG. 2B, form a continuous nanostructured block copolymermaterial, according to an embodiment of the invention.

FIG. 3A is a schematic drawing of a triblock copolymer molecule thatincludes three different polymer blocks, according to an embodiment ofthe invention.

FIG. 3B is a schematic drawing that shows how multiple triblockcopolymer molecules, as shown in FIG. 3A, arrange themselves into adomain structure, according to an embodiment of the invention.

FIG. 3C is a schematic drawing that shows how multiple repeat domains,as shown in FIG. 3B, form a continuous nanostructured block copolymermaterial, according to an embodiment of the invention.

FIG. 4 is a schematic illustration of a lithium metal cell with asulfur-based cathode that uses a sulfur-sequestering catholyte,according to an embodiment of the invention.

FIG. 5 is a schematic illustration of a lithium metal cell with asulfur-based cathode that has a layer of sulfur-sequestering electrolytebetween the cathode and the separator, according to an embodiment of theinvention.

FIG. 6 is a schematic illustration of a lithium metal cell with asulfur-based cathode in which individual cathode active materialparticles are coated with a sulfur-sequestering electrolyte, accordingto an embodiment of the invention.

FIG. 7 is a cross-sectional schematic drawing of an electrochemicalcell, according to an embodiment of the invention.

FIG. 8 is a cross-sectional schematic drawing of an electrochemicalcell, according to another embodiment of the invention.

SUMMARY

A new polymer composition is disclosed in the embodiments of theinvention. The new composition is an ionically conductive polymer thatincludes an electrophilic group capable of or configured to undergonucleophilic substitution. The polymer may have at least two differentmonomers, wherein a first monomer is ionically conductive and a secondmonomer comprises the electrophilic group capable of nucleophilicsubstitution. The ionically conductive monomer may be any of ethyleneoxides, acrylonitriles, phosphoesters, ethers, amines, imides, amides,alkyl carbonates, nitriles, siloxanes, phosphazines, olefins, dienes,and combinations thereof. The electrophilic group may be any of alkylhalides, alkyl sulfonates, alkyl phosphates, alkyl carbonates, oxiranes,aryl halides, and/or aryl sulfonates. The polymer may be used as anelectrolyte with the addition of an electrolyte salt, such as a lithiumsalt.

The polymer composition may combine with elemental sulfur, carbon, and ametal salt to form a cathode. The elemental sulfur may also have one ormore additives, such as carbon, silica, aluminum oxide, and titaniumdioxide, to form a sulfur composite. There may also be a currentcollector in electrical communication with the cathode.

In one embodiment of the invention, an electrochemical cell has acathode as described above that includes a Li salt, a lithium metalanode, and a separator between the cathode and the anode. The separatorprovides a path for ionic conduction between the cathode and the anode.In one arrangement, there is also a layer of the polymer compositiondescribed above between the cathode and the separator.

In another embodiment of the invention, a block copolymer electrolytehas a first lamellar domain comprising a plurality of first polymerblocks made from the polymer composition described above and a salt anda second lamellar adjacent to the first lamellar domain and comprising aplurality of second polymer blocks the second domain. The first domainforms a conductive portion of the electrolyte material. The seconddomain forms a structural portion of the electrolyte material.

The first lamellar domain and the second lamellar domain may comprise aplurality of linear diblock copolymers. The linear diblock copolymer mayhave a molecular weight of at least 150,000 Daltons or at least 350,000Daltons.

The first lamellar domain and the second lamellar domain may comprise aplurality of linear triblock copolymers.

The second polymer blocks may comprise a non-ionic-conducting polymerwith a bulk modulus greater than 10⁷ Pa at 90 degrees C. The secondpolymer blocks may comprise a component selected from a group comprisingstyrene, methacrylate, vinylpyridine, vinylcyclohexane, imide, amide,propylene, alphamethylstyrene and combinations thereof.

An electrochemical cell is disclosed. The cell has a cathode thatcontains at least a SPAN cathode active material, an electronicallyconducting agent, and a first polymer electrolyte that contains alithium salt, all mixed together. The cell has a lithium anode and aseparator positioned between the cell and the anode. The anode may be alithium metal film.

In one arrangement, the first polymer electrolyte is a liquid and thecell also contains a binder. In another arrangement, the first polymerelectrolyte is a solid polymer electrolyte. The separator may contain asecond solid polymer electrolyte.

At least one of the first polymer electrolyte and the separator may beconfigured to react chemically with elemental sulfur. At least one ofthe first polymer electrolyte and the separator may contain aradical-generating species, such as bromine, TEMPO groups or pendantmethacrylate groups.

At least one of the first polymer electrolyte and the separator may beconfigured to react chemically with lithium polysulfide. At least one ofthe first polymer electrolyte and the separator may include anelectrolyte salt and an ionically conductive polymer that includes anelectrophilic group capable of nucleophilic substitution or an ionicallyconductive polymer that includes an olefinic group capable ofpolysulfide addition.

At least one of the first polymer electrolyte and the separator may beconfigured to sequester sulfur by physical interaction. At least one ofthe first polymer electrolyte and the separator may contain a linearcopolymer of carbonates. ethylene oxide (P(LC-EO)), and P(LC-EO)s whichincorporate thioethers linkages in addition to ether linkages(P(LC-TEO)). At least one of the first polymer electrolyte and theseparator may contain a molecule that has a polymer backbone to whichpolar groups are attached. At least one of the first polymer electrolyteand the separator may contain a molecule that has a polyether backbonewith cyclic carbonates grafted as side groups (P(GC-EO)). The polymerbackbone may be any of (P(GN-EO) or P(GP-EO)), polyalkanes,polyphosphazenes, or polysiloxanes. The polar groups may be any ofnitrile groups (GN), phosphonate groups (GP), prises poly phosphorusesters.

At least one of the first polymer electrolyte and the separator may beconfigured to sequester lithium polysulfide by physical interaction. Inone arrangement, at least one of the first polymer electrolyte and theseparator contains a polyelectrolyte, such as a cationic polymer withcounterions such as any of Cl⁻, TFSI⁻, BETI⁻, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻,Triflate⁻, and BOB⁻. In another arrangement, at least one of the firstpolymer electrolyte and the separator contains an anionic polymer, suchas any of Nafion®, poly(styrene sulfonate), polyvinyl and sulfonate.

In one embodiment of the invention, at least one of the first polymerelectrolyte and the separator contains a block copolymer that has afirst lamellar domain made of first polymer blocks that contain anionically conductive polymer that includes an electrophilic groupcapable of nucleophilic substitution and an electrolyte salt and asecond lamellar domain made of second polymer blocks, the second domainadjacent the first lamellar domain and forming a structural portion ofthe electrolyte material. The second polymer blocks may have a componentsuch as styrene, methacrylate, vinylpyridine, vinylcyclohexane, imide,amide, propylene, alphamethylstyrene and combinations thereof. The firstlamellar domain and the second lamellar domain may contain a pluralityof linear block copolymers, which may be diblock or triblock copolymers.

In one embodiment of the invention, a cathode contains at least SPANcathode active material, an electronically conducting agent, and a firstpolymer electrolyte with a lithium salt, all mixed together. There maybe a layer of a second polymer electrolyte on the surface of the cathodefilm that faces the separator layer in the battery. In one arrangement,the first polymer electrolyte and the second polymer electrolyte are thesame. There may also be a current collector on the surface of thecathode film that faces away from the separator.

The SPAN material may be mixed with one or more additives such as any ofcarbon, silica, aluminum oxide, and titanium dioxide, to form a sulfurcomposite.

The electronically conductive agent may be any of carbon black,graphite, conductive carbons, and conductive polymers. Exemplaryconductive polymers include polythiophene, polyphenylene vinylene,polypyrrole, polyphenylene sulfide, and cyclized polyacrylonitrile.

In one arrangement, the cathode contains no fluorinated polymers.

The first polymer electrolyte may be a solid block copolymer that iseither a diblock copolymer or a triblock copolymer. The first block ofthe diblock or triblock copolymer may be ionically conductive, suchpolyethers, polyamines, polyimides, polyamides, poly(alkyl carbonates),polynitriles, polysiloxanes, polyphosphazenes, polyolefins, polydienes,and combinations thereof. The first block of the diblock or triblockcopolymer may be an ionically-conductive comb polymer that has abackbone and pendant groups. The backbone may be any of polysiloxanes,polyphosphazines, polyethers, polydienes, polyolefins, polyacrylates,polymethacrylates, and combinations thereof. The pendants may be any ofoligoethers, substituted oligoethers, nitrile groups, sulfones, thiols,polyethers, polyamines, polyimides, polyamides, poly(alkyl carbonates),polynitriles, other polar groups, and combinations thereof.

The second block of the diblock or triblock copolymer may be any ofpolystyrene, hydrogenated polystyrene, polymethacrylate, poly(methylmethacrylate), polyvinylpyridine, polyvinylcyclohexane, polyimide,polyamide, polypropylene, polyolefins, poly(t-butyl vinyl ether),poly(cyclohexyl methacrylate), poly(cyclohexyl vinyl ether),poly(t-butyl vinyl ether), polyethylene, fluorocarbons, polyvinylidenefluoride, and copolymers that contain styrene, methacrylate, and/orvinylpyridine.

At least one of the first polymer electrolyte and the second polymerelectrolyte may sequester sulfur or lithium polysulfide by chemicalbonding. At least one of the first polymer electrolyte and the secondpolymer electrolyte may contain an ionically conductive polymer thatincludes an olefinic group capable of polysulfide addition and anelectrolyte salt. In one arrangement, the ionically conductive polymerincludes a first monomer and a second monomer: the first monomer isionically conductive and the second monomer contains an olefinic groupcapable of polysulfide addition. The ionically conductive monomer may beany of ethylene oxides, acrylonitriles, phosphoesters, ethers, amines,imides, amides, alkyl carbonates, nitriles, siloxanes, phosphazines,olefins, dienes, and combinations thereof. The olefinic group may be anallyl group, such as an allyloxymethyl and a vinyl group. In onearrangement, at least one of the first polymer electrolyte and thesecond polymer electrolyte contains a radical-generating species, suchas any of bromine, TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl)oxy or(2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl) groups, and pendantmethacrylate groups.

At least one of the first polymer electrolyte and the second polymerelectrolyte may be configured to sequester sulfur or lithium polysulfidephysically. At least one of the first polymer electrolyte and the secondpolymer electrolyte may contain a molecule that has a polymer backboneto which polar groups are attached. The backbone may be a polyethermolecule that has cyclic carbonates grafted as side groups (P(GC-EO)).The polymer backbone may be any of (P(GN-EO), P(GP-EO)), polyalkanes,polyphosphazenes, and polysiloxanes and the polar groups are can beeither nitrile groups (GN) or phosphonate groups (GP). The polar groupsmay be poly phosphorus esters. At least one of the first polymerelectrolyte and the second polymer electrolyte may contain a linearcopolymer of carbonates, ethylene oxide (P(LC-EO)), or analogs ofP(LC-EO) that incorporate thioethers linkages in addition to etherlinkages (P(LC-TEO)). At least one of the first polymer electrolyte andthe second polymer electrolyte may contain a polyelectrolyte, such asCl⁻, TFSI⁻, BETI⁻, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, Triflate⁻, or BOB⁻. At leastone of the first polymer electrolyte and the second polymer electrolytemay contain an anionic polymer, such as any of Nafion®, poly(styrenesulfonate), polyvinyl and sulfonate.

DETAILED DESCRIPTION

The preferred embodiments are illustrated in the context of lithiummetal-sulfur electrochemical cells. The skilled artisan will readilyappreciate, however, that the materials and methods disclosed hereinwill have application in a number of other contexts where diffusion ofsulfur or polysulfides is undesirable.

All publications referred to herein are incorporated by reference intheir entirety for all purposes as if fully set forth herein.

In the embodiments of the invention, a lithium metal battery that has asulfur-based cathode and a sulfur-sequestering electrolyte is disclosed.The electrolyte may be internal to the cathode or it may form a coatingbetween the cathode and the separator. The sulfur-sequesteringelectrolyte may act in any of a few different ways. It may preventformation and diffusion of unbound lithium polysulfide and/or elementalsulfur. The sequestration may be physical or it may be chemical.Beneficial aspects of such a battery include high specific capacity andlong cycle life.

In this disclosure, the terms “negative electrode” and “anode” are bothused to mean “negative electrode.” Likewise, the terms “positiveelectrode” and “cathode” are both used to mean “positive electrode.”

The term “separator” is used herein to mean either:

-   -   a permeable membrane placed between a battery's anode and        cathode, which is filled with a liquid or gel electrolyte, or    -   the region between a battery's anode and cathode, which is        filled with a solid electrolyte membrane.

The term “solid polymer” is used herein to include solid homopolymers,solid random copolymers and solid block copolymers.

High Energy Cell with Lithium Metal and SPAN Electrodes

In one embodiment of the invention, a cell design combines a lithiummetal anode, a block copolymer separator electrolyte, a compositecathode containing sulfur-bound cyclized polyacrylonitrile (SPAN), and acathode electrolyte (catholyte) that acts as a sulfur and/or polysulfidebarrier to prevent loss of elemental sulfur and/or lithium polysulfidesfrom the cathode layer.

Generally, a cathode has at least the following components:

-   -   an electrochemically active cathode material;    -   electronically conductive additives;    -   a polymeric binder to hold the active material and conductive        additives in place; and    -   a current collector backing the electrochemically active        material.

Examples of electronically conductive additives include carbon black,graphite, vapor-grown carbon fiber (VGCF), graphene, SuperP, Printex,Ketjenblack, and carbon nanotubes (CNTs). Examples of binders includepolyvinylidene fluoride (PVDF), poly(vinylidenefluoride-hexafluoropropylene) (PVDF-HFP), polyacrylonitrile (PAN),polyacrylic acid (PAA), alginate, polyethylene oxide (PEO),carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR),polypropylene oxide, and copolymers thereof. In a dry polymer battery,the polymeric binder may or may not be ionically conductive and may beaccompanied by an additional polymer electrolyte (catholyte) thatcontains a dissolved metal salt and functions as a metal iontransporter. For lithium batteries, lithium salts are used. Typicallithium salts include lithium bis(trifluoromethanesulfonyl) imide(LiTFSI), lithium bis(perfluoroethanesulfonyl) imide (LiBETI), LiClO₄,LiBF₄, LiPF₆, lithium trifluoromethanesulfonate (Li triflate), andlithium bis(oxalato) borate (LiBOB). Salts of other metals can be usedif other metals form the basis of the cell. Examples of such metalsinclude Na, K, Mg, Ca, and Al.

In a sulfur-based cell, the cathode is generally fabricated in thecharged state (oxidized) and requires a source of lithium. Anode choicesmay include lithium metal foil, lithiated graphite, lithiated silicon,or the like. Use of lithium metal anodes in a rechargeable cell requiresa specialized separator that is chemically stable to lithium as well ascapable of preventing dendritic growth during charging. Block copolymerelectrolytes provide a means of achieving the required mechanicalstrength and ionic conductivity.

The separator layer between the anode and cathode is an ionicallyconductive, but electronically insulative layer. Such a layer may be aliquid-electrolyte-soaked porous plastic membrane in conventionallithium-ion cells or a solid polymer electrolyte coating in dry polymercells. Combinations of these are also possible. For a polymerelectrolyte such as PEO that is a viscous liquid or gel with poormechanical properties, greater mechanical strength can be achieved byforming block copolymers that have a first PEO polymer block that isionically conductive and a second polymer block that ismechanically-stabilizing. In order for the second block to providemechanical stability, the cell is operated at a temperature below themelting temperature (T_(m)) for crystalline polymers or the glasstransition temperature (T_(g)) for amorphous polymers.

Nanostructured Block Copolymer Electrolytes

FIG. 1A is a simplified illustration of an exemplary diblock polymermolecule 100 that has a first polymer block 110 and a second polymerblock 120 covalently bonded together. In one arrangement both the firstpolymer block 110 and the second polymer block 120 are linear polymerblocks. In another arrangement, either one or both polymer blocks 110,120 has a comb (or branched) structure. In one arrangement, neitherpolymer block is cross-linked. In another arrangement, one polymer blockis cross-linked. In yet another arrangement, both polymer blocks arecross-linked.

Multiple diblock polymer molecules 100 can arrange themselves to form afirst domain 115 of a first phase made of the first polymer blocks 110and a second domain 125 of a second phase made of the second polymerblocks 120, as shown in FIG. 1B. Diblock polymer molecules 100 canarrange themselves to form multiple repeat domains, thereby forming acontinuous nanostructured block copolymer material 140, as shown in FIG.1C. The sizes or widths of the domains can be adjusted by adjusting themolecular weights of each of the polymer blocks.

In one arrangement the first polymer domain 115 is ionically conductive,and the second polymer domain 125 provides mechanical strength to thenanostructured block copolymer.

FIG. 2A is a simplified illustration of an exemplary triblock polymermolecule 200 that has a first polymer block 210 a, a second polymerblock 220, and a third polymer block 210 b that is the same as the firstpolymer block 210 a, all covalently bonded together. In one arrangementthe first polymer block 210 a, the second polymer block 220, and thethird copolymer block 210 b are linear polymer blocks. In anotherarrangement, either some or all polymer blocks 210 a, 220, 210 b have acomb (or branched) structure. In one arrangement, no polymer block iscross-linked. In another arrangement, one polymer block is cross-linked.In yet another arrangement, two polymer blocks are cross-linked. In yetanother arrangement, all polymer blocks are cross-linked.

Multiple triblock polymer molecules 200 can arrange themselves to form afirst domain 215 of a first phase made of the first polymer blocks 210a, a second domain 225 of a second phase made of the second polymerblocks 220, and a third domain 215 b of a first phase made of the thirdpolymer blocks 210 b as shown in FIG. 2B. Triblock polymer molecules 200can arrange themselves to form multiple repeat domains 225, 215(containing both 215 a and 215 b), thereby forming a continuousnanostructured block copolymer 230, as shown in FIG. 2C. The sizes ofthe domains can be adjusted by adjusting the molecular weights of eachof the polymer blocks.

In one arrangement the first and third polymer domains 215 a, 215 b areionically conductive, and the second polymer domain 225 providesmechanical strength to the nanostructured block copolymer. In anotherarrangement, the second polymer domain 225 is ionically conductive, andthe first and third polymer domains 215 provide a structural framework.

FIG. 3A is a simplified illustration of another exemplary triblockpolymer molecule 300 that has a first polymer block 310, a secondpolymer block 320, and a third polymer block 330, different from eitherof the other two polymer blocks, all covalently bonded together. In onearrangement the first polymer block 310, the second polymer block 320,and the third copolymer block 330 are linear polymer blocks. In anotherarrangement, either some or all polymer blocks 310, 320, 330 have a comb(or branched) structure. In one arrangement, no polymer block iscross-linked. In another arrangement, one polymer block is cross-linked.In yet another arrangement, two polymer blocks are cross-linked. In yetanother arrangement, all polymer blocks are cross-linked.

Multiple triblock polymer molecules 300 can arrange themselves to form afirst domain 315 of a first phase made of the first polymer blocks 310a, a second domain 325 of a second phase made of the second polymerblocks 320, and a third domain 335 of a third phase made of the thirdpolymer blocks 330 as shown in FIG. 3B. Triblock polymer molecules 300can arrange themselves to form multiple repeat domains, thereby forminga continuous nanostructured block copolymer 340, as shown in FIG. 3C.The sizes of the domains can be adjusted by adjusting the molecularweights of each of the polymer blocks.

In one arrangement the first polymer domains 315 are ionicallyconductive, and the second polymer domains 325 provide mechanicalstrength to the nanostructured block copolymer. The third polymerdomains 335 provides an additional functionality that may improvemechanical strength, ionic conductivity, chemical or electrochemicalstability, may make the material easier to process, or may provide someother desirable property to the block copolymer. In other arrangements,the individual domains can exchange roles.

Choosing appropriate polymers for the block copolymers described aboveis important in order to achieve desired electrolyte properties. In oneembodiment, the conductive polymer (1) exhibits ionic conductivity of atleast 10⁻⁵ Scm⁻¹ at electrochemical cell operating temperatures whencombined with an appropriate salt(s), such as lithium salt(s); (2) ischemically stable against such salt(s); and (3) is thermally stable atelectrochemical cell operating temperatures. In one embodiment, thestructural material has a modulus in excess of 1×10⁵ Pa atelectrochemical cell operating temperatures. In one embodiment, thethird polymer (1) is rubbery; and (2) has a glass transition temperaturelower than operating and processing temperatures. It is useful if allmaterials are mutually immiscible.

In one embodiment of the invention, the conductive phase can be made ofa linear or branched polymer. Conductive linear or branched polymersthat can be used in the conductive phase include, but are not limitedto, polyethers, polyamines, polyimides, polyamides, alkyl carbonates,polynitriles, and combinations thereof. The conductive linear orbranched polymers can also be used in combination with polysiloxanes,polyphosphazines, polyolefins, and/or polydienes to form the conductivephase.

In another exemplary embodiment, the conductive phase is made of comb(or branched) polymers that have a backbone and pendant groups.Backbones that can be used in these polymers include, but are notlimited to, polysiloxanes, polyphosphazines, polyethers, polydienes,polyolefins, polyacrylates, polymethacrylates, and combinations thereof.Pendants that can be used include, but are not limited to, oligoethers,substituted oligoethers, nitrile groups, sulfones, thiols, polyethers,polyamines, polyimides, polyamides, alkyl carbonates, polynitriles,other polar groups, and combinations thereof.

Further details about polymers that can be used in the conductive phasecan be found in International Patent Application Number PCT/US09/45356,filed May 27, 2009 (PCT Publication WO2009146340 published Dec. 3,2009), International Patent Application Number PCT/US09/54709, filedAug. 22, 2009 (U.S. Pat. No. 8,691,928 issued Apr. 8, 2014),International Patent Application Number PCT/US10/21065, filed Jan. 14,2010 (PCT Publication WO2010083325 published Jul. 22, 2010),International Patent Application Number PCT/US10/21070, filed Jan. 14,2010 (PCT Publication WO2010083330 published Jul. 22, 2010), U.S.International Patent Application Number PCT/US10/25680, filed Feb. 26,2009 (PCT Publication WO2010101791 published Sep. 10, 2010), andInternational Patent Application Number PCT/US10/25690, filed Feb. 26,2009 (U.S. Pat. No. 8,598,273 issued Dec. 3, 2013, all of which areincluded by reference herein.

There are no particular restrictions on the electrolyte salt that can beused in the block copolymer electrolytes. Any electrolyte salt thatincludes the ion identified as the most desirable charge carrier for theapplication can be used. It is especially useful to use electrolytesalts that have a large dissociation constant within the polymerelectrolyte.

Suitable examples include alkali metal salts, such as Li salts. Examplesof useful Li salts include, but are not limited to, LiPF₆, LiN(CF₃SO₂)₂,Li(CF₃SO₂)₃C, LiN(SO₂CF₂CF₃)₂, LiB(C₂O₄)₂, Li₂B₁₂F_(x)H_(12-x),Li₂B₁₂F₁₂, and mixtures thereof.

In one embodiment of the invention, single ion conductors can be usedwith electrolyte salts or instead of electrolyte salts. Examples ofsingle ion conductors include, but are not limited to sulfonamide salts,boron based salts, and sulfates.

In one embodiment of the invention, the structural phase can be made ofglassy or crystalline polymers such as polystyrene, hydrogenatedpolystyrene, polymethacrylate, poly(methyl methacrylate),polyvinylpyridine, polyvinylcyclohexane, polyimide, polyamide,polypropylene, poly (2,6-dimethyl-1,4-phenylene oxide) (PXE),polyolefins, poly(t-butyl vinyl ether), poly(cyclohexyl methacrylate),poly(cyclohexyl vinyl ether), polyethylene, fluorocarbons, such aspolyvinylidene fluoride, or copolymers that contain styrene,methacrylate, or vinylpyridine.

Additional species can be added to nanostructured block copolymerelectrolytes to enhance the ionic conductivity, to enhance themechanical properties, or to enhance any other properties that may bedesirable.

The ionic conductivity of nanostructured block copolymer electrolytematerials can be improved by including one or more additives in theionically conductive phase. An additive can improve ionic conductivityby lowering the degree of crystallinity, lowering the meltingtemperature, lowering the glass transition temperature, increasing chainmobility, or any combination of these. A high dielectric additive canaid dissociation of the salt, increasing the number of Li+ ionsavailable for ion transport, and reducing the bulky Li+[salt] complexes.Additives that weaken the interaction between Li+ and PEO chains/anions,thereby making it easier for Li+ ions to diffuse, may be included in theconductive phase. The additives that enhance ionic conductivity can bebroadly classified in the following categories: low molecular weightconductive polymers, ceramic particles, room temp ionic liquids (RTILs),high dielectric organic plasticizers, and Lewis acids.

Other additives can be used in the polymer electrolytes describedherein. For example, additives that help with overcharge protection,provide stable SEI (solid electrolyte interface) layers, and/or improveelectrochemical stability can be used. Such additives are well known topeople with ordinary skill in the art. Additives that make the polymerseasier to process, such as plasticizers, can also be used.

Further details about block copolymer electrolytes are described in U.S.patent application Ser. No. 12/225,934, filed Oct. 1, 2008 (U.S. Pat.No. 8,563,168 issued Oct. 22, 2013), U.S. patent application Ser. No.12/271,829, filed Nov. 14, 2008 (U.S. Pat. No. 8,268,197 issued Sep. 18,2012, and International Patent Application Number PCT/US09/31356, filedJan. 16, 2009 (U.S. Pat. No. 8,889,301 issued Nov. 18, 2014), all ofwhich are included by reference herein.

SPAN Cathode Active Material

If the cathode of a lithium-sulfur cell consisted completely ofelemental sulfur, in theory, an energy density of more than 1,000 Wh/kgcould be achieved. However, sulfur is neither ionically nor electricallyconductive, so a sulfur cathode includes additives that supply theseproperties and which lower the energy density. In addition, during thedischarge of a lithium-sulfur cell, elemental sulfur is usually reducedto soluble polysulfides. The polysulfides can diffuse into other regionsof the cell (and can even reach the anode), where they are no longerable to participate in the electrochemical reactions of subsequentcharge/discharge cycles. In addition, polysulfides may be dissolved inthe electrolyte, where they cannot be reduced further. Therefore, atpresent, the energy density of lithium-sulfur cells is much lower thanthe theoretical value, and is estimated to be between 400 Wh/kg and 600Wh/kg. Even worse, the service life of lithium-sulfur cells may belimited to as few as 100 complete cycles or even less. Cycle life may beaffected by diffusion of polysulfides from the cathode to the anodewhere they can react with the lithium metal anode and shorten its life.Sulfur utilization in lithium-sulfur battery cells can be significantlyincreased when the sulfur is bound to cyclized polyacrylonitrile. Thusit is advantageous to use polyacrylonitrile-sulfur (SPAN) composite as acathode active material.

In one embodiment, the cathode active material contains apolyacrylonitrile-sulfur (SPAN) composite. SPAN is a composite materialthat is produced by reacting polyacrylonitrile (PAN) with sulfur (S).SPAN material has sulfur-carbon bonds which can bond polysulfides to theSPAN polymer matrix. In such a SPAN composite, the sulfur is fixedlybonded to a polymer structure on a sub-nanometer/nanometer scale. Inaddition the sulfur is finely or homogeneously distributed within theSPAN structure. SPAN has been shown to offer good cycling stability witha high sulfur utilization rate. In addition, SPAN has shown such goodperformance even at high discharge rates (C rates).

In one embodiment of the invention, SPAN is produced by reactingpolyacrylonitrile with an excess of sulfur at a temperature greater thanor equal to 300° C. In some arrangements, temperatures greater than or550° C. are used. The ratio of excess sulfur to polyacrylonitrile thatis used depends on the reaction temperature. The sulfur atoms may be inthe polyacrylonitrile-sulfur composite material both directly bycovalent sulfur-carbon bonds, as well as indirectly through one or morecovalent bonds, sulfur-sulfur and one or more sulfur-carbon bonds may beconnected to a particular cyclized polyacrylonitrile. In this case, atleast a portion of the sulfur atoms of the SPAN composite material, forexample in the form of polysulfides, is covalently linked to a cyclizedpolyacrylonitrile. In such composite materials are indications ofsulfur-carbon bond, which tie the polysulfides to the polymer matrix.Consequently, there is a sulfur polyacrylonitrile composite havingvarious functional groups and chemical bonds, which can all havedifferent properties with respect to electrochemical performance, andaging behavior.

In another embodiment of the invention, SPAN composite material isproduced by (a) converting polyacrylonitrile to cyclizedpolyacrylonitrile, and (b) converting the cyclized polyacrylonitrilewith sulfur to form a polyacrylonitrile-sulfur composite material. Instep (a), an electrically conductive cyclized polyacrylonitrile (cPAN)base is formed. In step (b), the cPAN is reacted with electrochemicallyactive sulfur takes place, bonding the sulfur covalently to theelectrically conductive skeleton of the cPAN, thus formingpolyacrylonitrile-sulfur composite material (SPAN). By using a two-stepmethod, reaction conditions can be optimized for each partial reaction.It may be interesting to note that step (a) is similar to adehydrogenation reaction known from the preparation of carbon fiber, andstep (b) is similar to a reaction from a different technical field,namely the vulcanization reaction of rubber.

This cell design has numerous embodiments related to differentarchitectural configurations of a barrier catholyte as well as numerousmechanistic processes for preventing loss of sulfur and/or polysulfides.Different catholyte materials or additives can be used to capture sulfurspecies and prevent capacity fade.

The embodiments of the invention include various solid polymerelectrolyte materials that can be used as active or passive barriers toprevent sulfur/polysulfide loss. Such polymers may or may not becross-linked. In general the disclosed materials and related mechanismfor preventing sulfur loss can be classified into two groups: active (orchemical) and passive (or physical) barriers to preventsulfur/polysulfide loss. It should be understood that for any suchpolymer material disclosed below, various structural configurations arepossible. For example, the monomers that make up any such polymer may beorganized as random copolymers or in blocks to make block copolymerstructures. In addition, the polymers (and the monomers therein)disclosed below may be combined with yet other polymers to form randomcopolymer or block copolymer structures.

In a solid-state lithium polymer battery, electrophilic groups areincluded in the polymer used as the ionically conductive binder in asulfur-based cathode. During operation of the battery, the electrophilicgroups are positioned to rapidly react with any free lithium polysulfidespecies that are formed, forming an electrochemically stablecarbon-sulfur bond and a lithium salt and preventing further migrationof the polysulfide species. Depending on the identity of the polysulfidespecies, it may be able to continue lithium redox cycles at moderatelyreduced capacity relative to the original cathode; regardless, thepolysulfide species is prevented from diffusing to the anode and causinghigher internal resistance to ion flow.

In an operating lithium-sulfur cell, lithium polysulfides are formed asintermediates in the reduction of elemental sulfur in the cathode todilithium sulfide:S₈→Li₂S₈→Li₂S₆→Li₂S₄→Li₂S₃→Li₂S₂→Li₂S

The leftmost and rightmost species in this sequence (S₈, Li₂S₂, andLi₂S) have low solubilities in most ionically conductive media andtherefore form immobilized solid precipitates. The polysulfide speciesshown in bold tend to dissolve in ionically conductive media, and thendiffuse away in electrolyte from their point of origin in the cathode.When they diffuse far enough from the cathode into the separator betweenthe cathode and anode, they lose electrical contact with the cathode andcannot be reduced further, resulting in an irreversible loss of cellcapacity. If these species diffuse across the entire separator and reachthe anode, they are likely to be spontaneously, fully reduced todilithium sulfide. This reaction both degrades the anode material itselfand forms an inhibiting layer of precipitated dilithium sulfide at theanode surface, further degrading cell performance.

Prevention of the diffusion of polysulfide species is a major hurdle inthe development of lithium-sulfur batteries with long cycle life. Mostreported research involves the creation of sulfur composites that formthe active material of the cathode. Nanostructured layers of carbon,titanium dioxide, or other materials can form barriers around sulfurparticles that limit the diffusion of polysulfide species, increasingthe stability and lifetime of cells, though there is some appreciableloss of capacity due to the inclusion of inactive components in thecathode. Depending on the complexity of the nanostructured composites,scaling up to commercial quantities may be more or less practical andsuccessful in creating “contained sulfur.”

A cathode has of multiple components, the active material being onlyone. Generally, a cathode also has a polymeric binder to hold activematerial particles in place on a current collector backing, and anelectrically conductive additive such as carbon black, graphite, orvapor-grown carbon fiber (VGCF) to ensure electrical connectivity to allof the active material particles. In a dry polymer battery, thepolymeric binder must also function as a lithium ion transporter, andwill contain some amount of a dissolved lithium salt. The separatorlayer between the anode and cathode is also essential to the operationof a cell as an ionically conductive but electrically insulative layer.The separator may be a liquid-electrolyte-soaked porous plastic layer inconventional lithium-ion cells or a solid plastic coating in dry polymercells, or some combination thereof. All components of a lithium-sulfurcell may provide additional opportunities to stabilize the operation oflithium-sulfur batteries.

Active Barriers for Lithium Polysulfides

In one embodiment, a catholyte polymer is an active polysulfide barrierthat captures unbound lithium polysulfides by chemical reaction(actively). During operation of a battery cell, reactive groups of thecatholyte are available to react rapidly with any free lithiumpolysulfide species that are generated, forming stable carbon-sulfurbonds, thus sequestering the polysulfides and preventing migration ofthe polysulfide species away from the cathode. Depending on theparticular polysulfide species that are sequestered, there may still beenough capacity left in the cathode that the battery can continue tocycle at an acceptable, though reduced capacity relative to the originalcathode.

In one embodiment of the invention the catholyte is anepichlorohydrin-ethylene oxide copolymer (P(EC-EO)). The catholyte mayalso be used as a cathode binder. The structure of P(EC-EO) is:

wherein n is the total number of repeat units, and x is the molefraction of EO units, leaving 1−x as the mole fraction of EC units.Values of x can range from 0 to 0.99, and values of n can range fromabout 10 to 200,000 or greater. P(EC-EO) can be dissolved in commonorganic solvents, and the solution can be useful for forming coatings.The structure of P(EC-EO) represents a perturbation of the poly(ethyleneoxide) homopolymer, which is a known lithium ion conductor when mixedwith a lithium salt. The pendant chloromethyl groups derived from theepichlorohydrin monomers only modestly decrease the lithium ionconductivity of the polymer relative to PEO homopolymer, as long as themole ratio of EC (1−x) is kept low (e.g., 0<1−x<0.5).

The pendant chloromethyl group of the EC portion of P(EC-EO) iselectrophilic, meaning that a nucleophilic unit (“Nu-”) can displace achloride ion while forming a new bond to the adjacent carbon atom:

Unbound lithium polysulfide species generated during operation oflithium-sulfur batteries (general formula “Li_(m)S_(y)”, m=1 or 2, y=3to 8) are known to be good nucleophiles. If they are formed in thecathode in the presence of a P(EC-EO) binder, it is highly likely thatreactions similar to the following will take place (as a spectator ion,Li⁺ is not included in the reaction):

The substitution reaction is expected to be permanent: the C—S bond doesnot break under normal cell operating conditions. The polysulfidespecies has thus been trapped in the cathode, is still in electricalcontact with the cathode, and cannot migrate or diffuse to the anode.The formation of the C—S bond causes an irreversible loss in cathodecapacity as the sulfur can no longer be fully oxidized, but this loss issmaller than what would result from diffusion of the entire polysulfidespecies away from the cathode. Therefore, a sulfur or sulfur compositecathode formulated with some portion of a P(EC-EO) polymer has higherstability than one made without, due to the reduced diffusion of lithiumpolysulfide species away from the cathode.

While the above describes one example, there are other embodiments withstructures that satisfy the general criteria of an electrophilic groupsusceptible to nucleophilic substitution incorporated into an ionicallyconductive polymer. The P(EC-EO) structure can be generalized to

in which the electrophilic methyl group is substituted with, forexample, Z=chloride, bromide, iodide, methanesulfonate,p-methyltoluenesulfonate, or p-nitrobenzenesulfonate. These can besynthesized using precursors such as epichlorohydrin, epibromohydrin,and glycidol. Other halide-bearing structures exhibit a similarcapability to undergo nucleophilic substitution, such as vinyl-chloridemonomer groups.

In other embodiments, PEG-brush polyacrylates and polymethacrylates ofthe following structure satisfy similar criteria, wherein the PEG sidechains provide ionic conductivity and alternative side chains providethe electrophilic component:

in which R═H (acrylate) or Me (methacrylate), x is the mole ratio of thePEG monomer component ranging from values of 0 to 0.99, n has the samevalues as above, 2<k<50 is the number of repeat units in the PEG sidechain, and R^(Z) is an electrophilic group susceptible to substitutionby polysulfide such as: 2-chloroethyl, 2-bromoethyl,ω-chloropoly(ethylene glycol), ω-bromopoly(ethylene glycol),ω-methanesulfonatopoly(ethylene glycol),ω-(p-toluenesulfonato)poly(ethylene glycol), glycidyl, orω-glycidylpoly(ethylene glycol).

PEG-brush poly(vinyl ethers) of the following structure satisfy similarcriteria, wherein the PEG side chains provide ionic conductivity andalternative side chains provide the electrophilic component:

in which x is the mole ratio of the PEG monomer component having thesame values as above, n has the same values as above, 2<k<50 is thenumber of repeat units in the PEG side chain, and R^(Z) is anelectrophilic group susceptible to substitution by polysulfide such as:2-chloroethyl, 2-bromoethyl, ω-chloropoly(ethylene glycol),ω-bromopoly(ethylene glycol), ω-methanesulfonatopoly(ethylene glycol),ω-(p-toluenesulfonato)poly(ethylene glycol), glycidyl, orω-glycidylpoly(ethylene glycol).

PEG-brush polyphosphazenes and PEG-brush polycarbonates can followsimilar substitution patterns:

in which x, k, n, and R^(Z) are as defined previously, and R^(C) is ashort alkyl chain, a short alkyl chain bearing an acyl side group, or ashort PEG chain.

Polyphosphoesters can also be included of the type:

in which R=alkyl C₁-C4, R^(Z) and n are as defined previously, and x and1−x are the mole ratios of the monomers defined previously. Thesepolymers are differentiated by the reactivity of the phosphoesterbackbone, as a polysulfide nucleophile could, by substitution, cleaveC—O bonds in the backbone or the —OR group instead of reacting only withthe electrophilic R^(Z) side chain.

In another embodiment of the invention, polyacrylonitrile can becopolymerized with chlorinated vinyl monomers to make polymers similarto:

in which x and 1−x are the mole fractions of the monomers, in which xcan range from 0.01 to 1. The value n is defined previously. Thechloride group in the backbone can be displaced by a polysulfidenucleophile, while polyacrylonitrile is an ionically conductive polymer.

Several of the polymers given as examples above exist as viscous liquidsor gels with poor mechanical properties, especially if they are mixedwith a plasticizing lithium salt such as LiTFSI. If greater mechanicalstrength is desired, it is possible to form block copolymers wherein a1^(st) polymer sequence serves as the ionic conductor and polysulfidetrap as described above and a 2^(nd) polymer sequence serves as amechanical block. Examples of suitable mechanical blocks includepolystyrene, poly(methyl methacrylate), poly(cyclohexyl methacrylate).The polymers typically are chosen such that they have microphaseseparation behavior.

In another embodiment of the invention, any of the polymers discussedabove is made into an ionic conductor by formulating it with anappropriate salt. Examples include, but are not limited to, lithiumsalts such as LiTFSI, LiBETI, LiClO₄, LiBF₄, LiPF₆, Li triflate, LiBOB,LiPF₆, LiN(CF₃SO₂)₂, Li(CF₃SO₂)₃C, LiN(SO₂CF₂CF₃)₂, LiB(C₂O₄)₂,Li₂B₁₂F_(x)H_(12-x), Li₂B₁₂F₁₂, and mixtures thereof. If a metal otherthan Li is the basis of the cell, such as Na, K, Mg, Ca, or Al, otherappropriate salts can be used.

In one arrangement, the polymers described above are used as admixturesother known ionically conducting formulations of polymers and metalsalts.

In another arrangement, the polymers described above, when formulatedwith a metal salt, can be used as a separator in an electrochemical cellwith a sulfur-based cathode. Such a separator can improve cellperformance by limiting polysulfide migration to the anode, but mayallow polysulfides to diffuse away from electrical contact with thecathode which would be observed as an irreversible loss in cellcapacity.

Olefin containing polymers can also be included. In one example of theinvention the catholyte is an allyl glycidyl ether-ethylene oxidecopolymer (P(AGE-EO)). The structure of P(AGE-EO) is:

wherein n is the total number of repeat units having values definedabove, and x is the mole fraction of EO units leaving 1−x as the molefraction of AGE units, in which x can range from 0 to 0.99. Thestructure of P(AGE-EO) represents a perturbation of the poly(ethyleneoxide) homopolymer, which is a known lithium ion conductor when mixedwith a lithium salt. The pendant allyloxymethyl groups derived from theAGE monomers only modestly decrease the lithium ion conductivity of thepolymer relative to PEO homopolymer, as long as the mole ratio of AGE(1−x) is kept low (e.g. 0<1−x<0.5). It is an example of an olefin, thecommon name for the functional group consisting of 2 carbon atomsforming a double bond. Olefins are susceptible to a number of reactions,include radical-induced polymerization, addition reactions, andcycloaddition reactions.

Lithium polysulfide species are capable of addition reactions to olefinsas has been described in de Graaf, “Laboratory simulation of naturalsulphurization: I. Formation of monomeric and oligomeric isoprenoidpolysulphides by low-temperature reactions of inorganic polysulphideswith phytol and phytadienes,” Geochim. Cosmochim. Acta 1992, 56,4321-4328 and in de Graaf, “Low-temperature addition of hydrogenpolysulfides to olefins: formation of 2,2′-dialkyl polysulfides fromalk-1-enes and cyclic (poly)sulfides and polymeric organic sulfurcompounds from α,ω-dienes,” J. Chem. Soc. Perkin. Trans. 1 1995,635-640. This is similar to the industrial process of rubbervulcanization, in which sulfur is used to form crosslinks in naturallatex (polyisoprene) as described in Carroll, “Polysulfides—Nature'sorganic soluble sulfur,” Phosphorus, Sulfur and Silicon and RelatedElements 1994, 95, 517-518. If polysulfides are formed in the cathode inthe presence of a P(AGE-EO) binder, it is possible for reactions similarto the following to take place:

In this reaction, a portion of the polysulfide Li_(m)S_(y) (m=1 or 2,y=3 to 8) reacts to form a sulfur link between carbon atoms, with adepreciated lithium polysulfide being the co-product. The AGE-sulfuraddition reactions are expected to be permanent: a C—S bond does notbreak under normal cell operating conditions. A portion of thepolysulfide species has thus been trapped in the cathode, is still inelectrical contact with the cathode, and cannot migrate or diffuse tothe anode. The formation of the C—S bond causes an irreversible loss incathode capacity as the sulfur can no longer be fully oxidized, but thisloss is smaller than what would result from diffusion of the entirepolysulfide species away from the cathode. The lithium polysulfideco-product is more fully reduced than the original lithium polysulfideand is likely to be Li₂S₂ or Li₂S species, which have poor solubilityand mobility, and will therefore tend to stop diffusing out of thecathode and may be trapped by other mechanisms proposed herein.Therefore, a sulfur or sulfur composite cathode formulated with someportion of a P(AGE-EO) polymer may be expected to show higher stabilitythan one made without, due to the reduced diffusion of lithiumpolysulfide species away from the cathode.

In other embodiments of the invention, various structures that have anolefinic group capable of polysulfide addition incorporated into anionically conductive polymer can be used.

A person of ordinary skill in the art will understand that there aremany other structures that satisfy general criteria of an electrophilicgroup or an olefinic group susceptible to polysulfide additionincorporated into an ionically conductive polymer.

Several of the polymers given as examples above exist as viscous liquidsor gels with poor mechanical properties, especially if they are mixedwith a plasticizing lithium salt such as LiTFSI. If greater mechanicalstrength is desired, it is possible to form block copolymers wherein a1^(st) polymer sequence serves as the ionic conductor and polysulfide orsulfur trap as described above and a 2^(nd) polymer sequence serves as amechanical block. Examples of suitable mechanical blocks includepolystyrene, poly(methyl methacrylate), poly(cyclohexyl methacrylate).The polymers typically are chosen such that they have microphaseseparation behavior.

In another embodiment of the invention, an of the polymers discussedabove is made into a ionic conductor by formulating it with anappropriate salt. Examples include, but are not limited to, lithiumsalts such as LiTFSI, LiBETI, LiClO₄, LiBF₄, LiPF₆, Li triflate, LiBOB,LiPF₆, LiN(CF₃SO₂)₂, Li(CF₃SO₂)₃C, LiN(SO₂CF₂CF₃)₂, LiB(C₂O₄)₂,Li₂B₁₂F_(x)H_(12-x), Li₂B₁₂F₁₂, and mixtures thereof. In a metal otherthan Li is the basis of the cell, such as Na, K, Mg, Ca, Al, etc, otherappropriate salts can be used, in which case the polymer is formulatedwith a salt of the metal that is the basis of the cell.

In one arrangement, the polymers described above are used as admixturesother known ionically conducting formulations of polymers and metalsalts.

In another arrangement, the polymers described above, when formulatedwith a metal salt, can be used as a separator in an electrochemical cellwith a sulfur-based cathode. Such a separator can improve cellperformance by limiting polysulfide migration to the anode, but mayallow polysulfides to diffuse away from electrical contact with thecathode which would be observed as an irreversible loss in cellcapacity.

Active Barriers for Elemental Sulfur

In another embodiment, an additional active mechanism serves to captureelemental sulfur through chemical reaction (actively). If elementalsulfur is formed during charging of the battery, it can diffuse out ofthe cathode causing capacity loss and eventually react at the surface ofthe anode causing resistance increase and anode decomposition. Duringoperation of the battery cell, reactive groups of the catholyte arepositioned to react rapidly with any elemental sulfur species that aregenerated, forming stable carbon-sulfur bonds and preventing furthermigration of the sulfur species. Sulfur bound in the catholyte in thisway may still be available for further electrochemical activity duringcycling of the battery cell.

In one embodiment of the invention, the catholyte contains aradical-generating species that is active at operating temperatures ofthe cell. In one example, the catholyte contains bromine, TEMPO groupsor pendant methacrylate groups. Representative examples include polymerswith the following structures:

If elemental sulfur species form during operation of the cell, radicalgenerating species such as those described herein may react with theelemental sulfur creating C—S bonds thus preventing loss of the sulfurfrom the cathode. The formation of the C—S bond causes an irreversibleloss in cathode capacity as the sulfur can no longer be fully oxidized,but this loss is smaller than what would result from diffusion of theentire sulfur species away from the cathode. Therefore, a sulfur orsulfur composite cathode formulated with some portion of a radicalforming polymer may be expected to show higher stability than one madewithout such a polymer, due to the reduced diffusion of sulfur speciesaway from the cathode.

Passive Barriers for Lithium Polysulfides

In another embodiment, the cell uses a catholyte that acts as a passive(or physical) barrier to diffusion of lithium polysulfides. Duringdischarge of the battery, free lithium polysulfide species may begenerated. Due to polarity and/or specific interactions of the catholyteand the unbound lithium polysulfides, the catholyte limits thedissolution of the lithium polysulfide species forming it into aprecipitate and preventing migration of the polysulfide species out ofthe cathode. The choice of catholyte can also affect theregioselectivity of the reduction of sulfur species. For example, properchoice of catholyte may result in formation of low index lithiumpolysulfides that have poorer solubility, resulting in entrapment in thecathode layer.

In one embodiment of the invention the catholyte is a polyelectrolyte ora polymerized ionic liquid. Representative examples include cationicpolymers with the following structures, in which X is an anion such asCl⁻, TFSI⁻, BETI⁻, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, Triflate⁻, BOB⁻, and the like.

Alternative choices for catholytes that affect polysulfide formation anddiffusion are anionic polymers such as Nafion®, poly(styrene sulfonate),polyvinyl sulfonate, and the like. Representative examples includeanionic polymers with the following structures:

The advantage of passive barriers for lithium polysulfides as comparedto active barriers is that they do not reduce the capacity of thecathode, in that no reduction of the polysulfide occurs.

Passive Barriers for Elemental Sulfur

In another embodiment, the cathode uses a catholyte that acts as apassive barrier to diffusion of elemental sulfur. During charging of thebattery, free sulfur species S_(x) (e.g., elemental sulfur) may begenerated. Such a catholyte has low solubility for elemental sulfur, sothere is little dissolution of the elemental sulfur species. Insteadsulfur precipitates are formed, preventing migration of the sulfurspecies out of the cathode. Low solubility can be achieved if thecatholyte is highly polar. Polarity of materials may be quantified bytheir dielectric constant. For example a low polarity material may havea dielectric constant of around 8 or less, while a highly polar materialmay have a dielectric constant greater than about 30. In onearrangement, such polarity is effected when a high concentration of saltspecies is dissolved in the catholyte. In another arrangement, a polymerthat is intrinsically polar is used as the catholyte. For theembodiments of the invention, as disclosed herein, polar materials areconsidered to have low enough solubility of sulfur to be useful asphysical barriers to elemental sulfur diffusion when they have adielectric constant greater than 20, or greater than 25, or greater than30, or any range therein.

In one embodiment of the invention the catholyte is a linear copolymerof carbonates and ethylene oxide (P(LC-EO)). In another exampledifferent analogs of P(LC-EO) can be used which incorporate thioetherslinkages in addition to ether linkages (P(LC-TEO)). Examples of somestructure of these types are:

In another embodiment, polar groups are attached to a polymer backbone,resulting in increased polarity, which reduces elemental sulfursolubility. In one arrangement, the catholyte is a polyether backbonewith cyclic carbonates grafted as side groups (P(GC-EO)). In anotherarrangement, different polar groups such as nitrile groups (GN) orphosphonate groups (GP) are grafted off the backbone polymer (P(GN-EO)or P(GP-EO)). Alternative backbones such as polyalkanes,polyphosphazenes, or polysiloxanes may be used. Representative examplesof structure of these types include:

in which n and x have the values as described above.

In another embodiment, poly phosphorus esters are used as catholytes tolimit elemental sulfur dissolution and diffusion. Representativeexamples of different structure of these types are:

in which R=methyl, ethyl, isopropyl, 2,2,2-trifluoroethyl, etc.Cell Architecture and Barrier Configurations

A barrier layer preventing sulfur loss from a cathode can be configuredin a variety of ways. A catholyte material which prohibits diffusion oflithium polysulfides or elemental sulfur may be located in differentregions of the cathode depending on its properties like conductivity,binding ability, and surface compatibility with other cell components.

FIG. 4 is a schematic illustration of a lithium metal cell 400 with asulfur-based cathode 420 that uses a sulfur-sequestering catholyte 430,according to an embodiment of the invention. The cell 400 also has alithium metal anode 440 and a separator 450 between the anode 440 andthe cathode 420. The sulfur-sequestering catholyte 430 is included inthe bulk of the cathode 420 and can be seen as light grey stripes. Thecathode 420 also has sulfur-containing active material particles 423 andelectronically conductive particles 426. In one arrangement, the cathodeactive material particles 423 are made of SPAN. The cathode 420 may alsocontain additional electrolytes or binders (not shown). There may alsobe a current collector 460 adjacent to the cathode 420.

FIG. 5 is a schematic illustration of a lithium metal cell 500 with asulfur-based cathode 520 that uses a sulfur-sequestering electrolyte,according to another embodiment of the invention. The cell 500 also hasa lithium metal anode 540 and a separator 550 between the anode 540 andthe cathode 520. There is a layer of sulfur-sequestering electrolyte 530between the separator 550 and the cathode 520 and can be seen as lightgrey stripes. The cathode 520 has sulfur-containing active materialparticles 523 and electronically conductive particles 526. In onearrangement, the cathode active material particles 523 are made of SPAN.The cathode 520 also contains a second electrolyte 529 and may containbinders (not shown). There may also be a current collector 560 adjacentto the cathode 520.

FIG. 6 is a schematic illustration of a lithium metal cell 600 with asulfur-based cathode 620 that uses a sulfur-sequestering catholyte,according to another embodiment of the invention. The cell 600 also hasa lithium metal anode 640 and a separator 650 between the anode 640 andthe cathode 620. The cathode 620 has sulfur-containing active materialparticles 623 and electronically conductive particles 626. In onearrangement, the cathode active material particles 623 are made of SPAN.The sulfur-containing active material particles 623 are coated with alayer of sulfur-sequestering catholyte 630, which can be seen as darkgrey edges 630 around the particles 623. In some arrangements, a chargetransfer tie-layer (not shown) may be used to improve charge transferbetween the catholyte coating 630 and the cathode particles 623. Thecathode 620 also contains a second electrolyte 629 and may containbinders (not shown). There may also be a current collector 660 adjacentto the cathode 620.

In another embodiment, two or more of the barrier configurations, asshown in FIGS. 4, 5, and 6 are used in the same cell. For example, acatholyte particle coating may be used to minimize the amount ofelemental sulfur that is formed. In addition a catholyte binder may beused to act as a barrier to elemental sulfur diffusion. Finally, anovercoat layer may be used to prevent lithium polysulfide and elementalsulfur diffusion.

FIG. 7 is a cross-sectional schematic drawing of an electrochemical cell702 with a positive electrode assembly 700, according to an embodimentof the invention. The positive electrode assembly 700 has a positiveelectrode (cathode) film 710 and an optional current collector 740. Thepositive electrode film 710 has positive electrode active materialparticles 720, such as elemental sulfur or a sulfur composite material,embedded in a matrix of electrolyte 730 that also contains small,electronically-conductive particles (as indicated by small grey dots)such as carbon black. The polymer electrolyte 730 can be a polymer or ablock copolymer, as described above. Combinations of the polymer andblock copolymer electrolyte are also possible. There is an optionalpositive electrode current collector 740 that may be a continuous orreticulated metal film. There is a negative electrode (anode) 760 thatis a metal layer, such as a lithium layer, that acts as both negativeelectrode active material and negative electrode current collector.

There is a separator region 750 filled with an electrolyte that providesionic communication between the positive electrode film 710 and thenegative electrode 760. In one arrangement, the separator region 750contains a solid electrolyte and can be the same electrolyte (withoutthe carbon particles) 730 as is used in the positive electrode film 710.

FIG. 8 is a cross-sectional schematic drawing of an electrochemical cell802 with a positive electrode assembly 800, according to anotherembodiment of the invention. The positive electrode assembly 800 has apositive electrode film 810 and an optional current collector 840. Thepositive electrode film 810 has positive electrode active materialparticles 820, such as elemental sulfur or a sulfur composite material,embedded in a matrix of electrolyte 830 that also contains small,electronically-conductive particles (as indicated by small grey dots)such as carbon black. The polymer electrolyte 830 can be a polymer or ablock copolymer, as described above. Combinations of the polymer andblock copolymer electrolyte are also possible. There is an optionalpositive electrode current collector 840 that may be a continuous orreticulated metal film. There is a negative electrode 860 that is ametal layer, such as a lithium layer, that acts as both negativeelectrode active material and negative electrode current collector.

There is a separator region 850 that has two regions, 850 a and 850 b.Region 850 a is a layer of a polymer or a block copolymer, as describedabove, and provides extra protection against diffusion of polysulfidespecies away from the cathode 810. When such a protective layer 850 a isused, it is possible to use a different kind of electrolyte within thecathode 810 itself, as the protective layer 850 a may be able to preventall polysulfide species from leaving the cathode 810 by itself. Region850 b is filled with an electrolyte that provides ionic communicationbetween the positive electrode film 810 (through the protective layer850 a) and the negative electrode 860. In one arrangement, the separatorregion 850 contains a solid electrolyte and can be the same electrolyte(without the carbon particles) 830 as is used in the positive electrodefilm 810 and or as used in the protective layer 850 a.

This invention has been described herein in considerable detail toprovide those skilled in the art with information relevant to apply thenovel principles and to construct and use such specialized components asare required. However, it is to be understood that the invention can becarried out by different equipment, materials and devices, and thatvarious modifications, both as to the equipment and operatingprocedures, can be accomplished without departing from the scope of theinvention itself.

We claim:
 1. An electrolyte comprising: a polymer that comprises: afirst monomer that is ionically conductive; and a second monomer thatincludes an electrophilic group capable of nucleophilic substitution;and an electrolyte salt; wherein the polymer comprises:

n is the total number of repeat units; x is the mole fraction of the PEGmonomer component k is the number of repeat units in the PEG side chain;R^(C) is selected from the group consisting of alkyl chains, alkylchains bearing an acyl side group, and PEG chains; and R^(Z) is anelectrophilic group susceptible to substitution by polysulfide and isselected from the group consisting of 2-chloroethyl, 2-bromoethyl,ω-chloropoly(ethylene glycol), ω-bromopoly(ethylene glycol),ω-methanesulfonatopoly(ethylene glycol),ω-(p-toluenesulfonato)poly(ethylene glycol), glycidyl, andω-glycidylpoly(ethylene glycol).
 2. The polymer of claim 1 wherein theelectrolyte salt is a lithium salt.
 3. A cathode comprising: elementalsulfur; carbon; and the electrolyte of claim
 1. 4. The cathode of claim3 wherein the elemental sulfur is mixed with one or more additivesselected from the group consisting of carbon, silica, aluminum oxide,and titanium dioxide to form a sulfur composite.
 5. The cathode of claim3 further comprising a current collector in electrical communicationwith the cathode.
 6. An electrochemical cell comprising: a cathodecomprising; elemental sulfur; carbon; and the electrolyte of claim 1; alithium metal anode; and a separator between the cathode and the anode,the separator providing a path for ionic conduction between the cathodeand the anode.
 7. The electrochemical cell of claim 6 further comprisinga layer of the electrolyte between the cathode and the separator.
 8. Ablock copolymer electrolyte comprising: a first lamellar domaincomprising a plurality of first polymer blocks comprising the polymer ofclaim 1; and an electrolyte salt; wherein the first lamellar domainforms a conductive portion of the electrolyte material; and a secondlamellar domain comprising a plurality of second polymer blocks, thesecond lamellar domain adjacent to the first lamellar domain; whereinthe second lamellar domain forms a structural portion of the electrolytematerial.
 9. The block copolymer electrolyte of claim 8 wherein thefirst lamellar domain and the second lamellar domain comprise aplurality of linear diblock copolymers.
 10. The block copolymerelectrolyte of claim 9 wherein the linear diblock copolymer has amolecular weight of at least 150,000 Daltons.
 11. The block copolymerelectrolyte of claim 9 wherein the linear diblock copolymer has amolecular weight of at least 350,000 Daltons.
 12. The block copolymerelectrolyte material of claim 8 wherein the first lamellar domain andthe second lamellar domain comprise a plurality of linear triblockcopolymers.
 13. The block copolymer electrolyte of claim 8 wherein thesecond polymer blocks comprise a non-ionic-conducting polymer with abulk modulus greater than 10⁷ Pa at 90 degrees C.
 14. The blockcopolymer electrolyte of claim 8 wherein the second polymer blockscomprise a component selected from a group comprising styrene,methacrylate, vinylpyridine, vinylcyclohexane, imide, amide, propylene,alphamethylstyrene and combinations thereof.